Spectacles are the traditional way of correcting defective vision in the human eye. However, refractive surgery which corrects defective vision by altering the cornea is now also increasingly being used. The aim of the surgical methods is to selectively alter the cornea so as to influence refraction. Differing procedures of surgeries are known for this purpose. Currently the most widespread is the so-called laser-assisted in situ keratomileusis, also abbreviated to LASIK. Firstly, a lamella of the cornea is cut on one side from the cornea surface and folded to the side. This lamella can be cut by means of a mechanical microkeratome or also by means of a so-called laser keratome, such as is marketed e.g. by Intralase Corp., Irvine, USA. After the lamella has been cut and folded to the side, the LASIK operation uses an excimer laser, which removes the thus-exposed corneal tissue by ablation. After volume in the cornea has been vaporized in this manner the lamella of the cornea is folded back into its original place.
The use of a laser keratome to expose the lamella is advantageous as the danger of infection is thereby reduced and the cut quality increased. In particular the lamella can be produced with a very much more consistent thickness. The cut is also potentially smoother, which reduces sight problems due to this boundary surface which remains even after the operation.
To produce the cut, a series of incisions of the eye are made at predetermined points such that the cut surface is formed as a result. With the laser keratome the cut surface forms the lamella to be folded back before the use of laser ablation.
With the conventional LASIK method exposed corneal tissue is vaporized, which is also called “grinding” of the cornea by means of laser radiation. The volume removal which is necessary to correct defective vision is set for each surface element of the exposed cornea by the number of laser pulses and their energy. Therefore, in the LASIK method, a so-called shot file is provided for the ablation laser which defines, for different points on the cornea, how often, and with what energy, the laser beam is to be directed onto defined points on the cornea. The volume removal is heuristically determined, not least because it depends greatly on the ablation effect of the laser beam, therefore on the wavelength, fluence etc. of the radiation used. The state of the cornea also plays a role; in particular the moisture content of the cornea is to be mentioned here. WO 96/11655 describes a device and a process for the LASIK method. In particular a formula is given which calculates the radius of curvature to be achieved from the pre-operative radius of curvature of the cornea and the desired diopter correction. A similar calculation is described in EP 1153584 A1—also for corneal ablation by means of LASIK.
U.S. Pat. No. 5,993,438 proposes the removal of a volume from the cornea by vaporization and absorption in the cornea.
WO 2005/092172 discloses how optical refraction power measurements which have been determined in one plane can be transferred into another plane. The document mentions that this process can be used for different eye treatments, in particular for laser-supported ablation.
A further laser-based eye surgery method is not to vaporize the volume to be removed from the cornea, but to isolate it by a laser cut. The volume is thus no longer ablated, but isolated in the cornea by a three-dimensional cut surface and thus made removable. Empirical values which have been developed for grinding the cornea by means of ablation laser radiation cannot be used for such methods. Instead, control data are required to operate the laser for isolating the volume to be removed from the cornea. One such procedure for eye surgery is described in U.S. Pat. No. 6,110,166 and U.S. Pat. No. 7,131,968. Different volume forms are shown in U.S. Pat. No. 6,110,166 and it is mentioned that the proper volume can be chosen by a person skilled in the art.
DE 102006053118 A1 describes the production of control data for the volume-isolating correction of defective vision.
It is known from DE 102006053120 A1 and DE 102006053119 A1 from Carl Zeiss Meditec AG to base the production of such defective vision on data which give the optical refraction power of spectacles suitable for correcting defective vision. It is also known from this published document, which thus describes a method of the mentioned type and a device of the mentioned type, to use data which also bring about a correction of an astigmatism or corrections of higher-order aberrations. By using data for defective vision which are intended for a conventional spectacle correction, the approach known from DE 102006053120 A1 achieves a considerable simplification in pre-operative eye measurement, as the production of spectacle correction data is daily practice in ophthalmology. However, this simplification also means a degree of limitation of the possible correction results, because inevitably only corrections which would also be possible with normal spectacles can be achieved. It is also to be taken into account here that corrections such as are possible e.g. with varifocals are ruled out for the approach according to DE 102006053120 A1 as such corrections always assume that, depending on the viewing direction, the axis of vision passes through the spectacle lens at different points, which makes it possible to be able to bring different optical properties of the spectacles to bear for different viewing directions (e.g. reading directed more downwards, or viewing directed more into the distance). This does not apply in the case of refractive surgery on the cornea because movement of the eye obviously causes the cornea to move as well when the direction of viewing changes. Thus, unlike with a spectacle lens, there is no change in the point where the optical axis penetrates the cornea when the eyeball rotates. The approach known from DE 102006053120 A1 can thus consequently use only comparatively simple spectacle defective-vision correction data as an input variable for control data, with the consequence of correspondingly limited possibilities of correction.
The precision with which the necessary cut surfaces are produced is of great importance for volume-isolating correction of defective vision. Unlike with a laser keratome, the position of the cut surfaces has a direct effect on the quality of the optical correction. With the conventional LASIK method, on the other hand, the precision with which the laser ablation is carried out is the only important factor determining the quality of the optical correction. This can already be seen from the fact that the cornea lamella is or has been produced in a large number of operations with a relatively crudely operating mechanical knife.
As the exact positioning of the eye is important for the precision production of the cut surfaces, the state of the art, for example WO 2005/011547 A1, proposes that a contact glass, against which the cornea is pressed, can be used in laser-surgery devices. This contact glass serves to fix the eye.
However, the precise position of the eye is not the only important factor for the precision of the cut surfaces; the shape of the cornea must also be known. As this varies from patient to patient within specific ranges, the contact glass also serves to give the cornea front surface a fixed, defined shape. When pressing the front of the cornea against the contact glass, there is consequently a deformation of the cornea which varies in size, depending on the deviation of the curvature of the contact glass from the natural curvature of the cornea of the respective patient.
If the position of the cut surfaces is important for the optical correction, i.e. if not just a lamella is isolated and the volume to be removed is removed by ablation, the deformation of the cornea is essential when determining the target coordinates for producing the cut surfaces. Therefore it is known in the state of the art to take into account the deformation by subjecting the previously determined target points to a coordinate transformation. In the named WO publication, this transformation is called a “contact pressure transformation” and there are transformation equations for a combination of spherical contact glass and spherical cornea front surface. DE 102008017293 A1 from Carl Zeiss Meditec AG complements these transformation equations with the result that coordinate transformation can also be carried out on different types of contact glasses and cornea curvatures.
The invention thus relates to the concept of carrying out a correction of the optical imaging errors of the human eye by cutting, by means of laser radiation within the cornea, a volume of tissue which is then removed from the cornea. A selective change of the optical refraction power of the cornea is thereby achieved. This change is localized, i.e. in the area of the cornea from which the tissue volume is removed. The pupil of the eye is usually taken as a basis.
The removal of the cut volume changes the geometry, i.e. the curvature of the cornea surface. In order that a desired correction of defective vision is achieved, the cut volume to be removed must therefore have special properties with regard to its shape.
The cut volume is usually circumscribed by three boundary surfaces, based on classic LASIK methods. An anterior boundary surface is formed at a constant distance under the cornea. This is particularly simple if the cornea is flattened by a flat contact glass. As this cut surface directionally lies furthest forward it is called anterior surface or, on the basis of the known LASIK methods, flap surface.
Furthermore, the volume is limited by a deeper-lying cut surface which is called posterior cut surface or, because the volume can be seen as a lenticle, as lenticle surface. Therefore, it is ensured that overall the volume to be removed changes the curvature of the cornea front surface. One of the two surfaces, usually the posterior one, generally has a geometry which is decisive for correcting defective vision.
In principle, it could be conceived to design the anterior and posterior surfaces such that they have a common cutting line. Firstly, this is not possible when correcting long-sightedness as there the volume to be removed must be thinner in the centre, i.e. in the area of the axis of vision, than at the edge. Secondly, when correcting farsightedness it might also be wished, for operational reasons, to ensure a certain minimum thickness of the volume at the edge in order to be able to remove it easily. The anterior surface and the posterior surface are therefore connected via a so-called lenticle edge surface.